8. DARK ENERGY PROJECTS

A diverse and ambitious set of projects to probe dark energy are in
progress or being planned. Here we provide a brief overview
of the observational landscape.
With the exception of experiments at the LHC
that might shed light on dark energy through discoveries about supersymmetry
or dark matter, all planned experiments involve cosmological observations.
Table 3
provides a representative sampling, not a comprehensive listing, of
projects that are currently proposed or under construction and does not
include experiments that have already
reported results. All of these projects share the common feature of
surveying wide areas to collect large samples of objects —
galaxies, clusters, or supernovae.

The Dark Energy Task Force (DETF) report
[Albrecht et al. 2006]
classified dark energy surveys into
an approximate sequence: on-going projects, either taking data or soon
to be taking data, are Stage II; near-future, intermediate-scale
projects are Stage III; and larger-scale, longer-term future projects
are designated Stage IV. More advanced stages are in general expected to
deliver tighter dark energy constraints, which the DETF quantified using
the w0 - wa figure of merit
(FoM) discussed in the Appendix
(Section 11.1). Stage III experiments
are expected to deliver
a factor ~ 3-5 improvement in the DETF FoM compared to the
combined Stage II results, while Stage IV experiments should improve the
FoM by roughly a factor of 10 compared to Stage II, though these
estimates are only indicative and are subject to considerable
uncertainties in systematic errors (see
Fig. 16).

We divide our discussion into ground- and space-based
surveys. Ground-based projects are typically less expensive than their
space-based counterparts and can employ larger-aperture telescopes. The
discovery of dark energy and many of the subsequent observations to date
have been dominated by ground-based telescopes. On the other hand, HST
(high-redshift SN observations), Chandra (X-ray clusters), and WMAP CMB
observations have played critical roles in probing dark energy. While
more challenging to execute, space-based surveys offer the advantages of
observations unhindered by weather and by the scattering, absorption,
and emission by the atmosphere, stable observing platforms free of
time-changing gravitational loading, and the ability to continuously
observe away from the sun and moon. They therefore have the potential
for much improved control of systematic errors.

A number of projects are underway to detect clusters and probe dark
energy using the SZE (see Sec.7.2).
These surveys are
coordinated with optical surveys that can determine cluster redshifts. The
Atacama Pathfinder EXperiment (APEX) survey in Chile
will cover up to 1000 square degrees. The largest of these projects
are the Atacama Cosmology Telescope (ACT) and the South Pole Telescope
(SPT), the latter of which will carry out a 4,000 square degree survey.

A number of optical imaging surveys are planned or proposed which can
study dark energy through weak lensing, clusters, and angular BAO using
a single wide-area survey. These projects use telescopes of intermediate
to large aperture and wide field-of-view, gigapixel-scale CCD cameras,
and are deployed at the best astronomical sites in order to obtain deep
galaxy photometry and shape measurements. They deliver
photometric-redshift information through color measurements using
multiple passbands. The ESO VLT Survey Telescope (VST) on Cerro Paranal
will carry out public surveys, including the 1500 sq. deg. KIDS survey
and a shallower, 5000 sq. deg. survey (ATLAS). The Panoramic Survey
Telescope and Rapid Response System (Pan-STARRS)-1 uses a 1.8-m
wide-field telescope to carry out several wide-area surveys from
Haleakala; in the future, they hope to deploy 4 × 1.8-m telescopes at
Mauna Kea in Pan-STARRS-4. The Dark Energy Survey (DES) will use a new 3
sq. deg. imager with red-sensitive CCDs on a 4-m telescope at Cerro
Tololo Inter-American Observatory (CTIO) in Chile to carry out a 5,000
sq. deg. survey in 5 optical passbands, covering the same survey area as
the SPT and partnering with the ESO VISTA Hemisphere Survey which will
survey the same area in 3 near-infrared bands. Hyper Suprime-Cam is a
new wide-field imager planned for the Subaru telescope on Mauna Kea that
will be used to carry out a deep survey over 2000 sq. deg. The Advanced
Liquid-mirror Probe of Asteroids, Cosmology and Astrophysics (ALPACA) is
a proposed rotating liquid mercury telescope that would repeatedly
survey a long, narrow strip of the sky at CTIO. The most ambitious of
these projects is the Large Synoptic Survey Telescope (LSST), which
would deploy a multi-Gigapixel camera with 10 sq. deg. field-of-view on
a new telescope on Cerro Pachon in Chile to survey 15,000 sq. deg. over
10 years.

Several large spectroscopic surveys have been designed to detect baryon
acoustic oscillations by measuring ~ 105 - 109
galaxy and QSO redshifts
using large multi-fiber spectrographs. WiggleZ is using the Anglo-Australian
Telescope to collect spectra of 400,000 galaxies in the redshift range
0.5 < z < 1. The Baryon Oscillation Sky Survey (BOSS)
proposes to use the SDSS telescope in New Mexico to measure galaxy
spectra out to z = 0.6. The Hobby
Eberly Telescope Dark energy EXperiment (HETDEX) plans to target
Ly-
emitters at higher redshift, 2
z 4. The Wide-Field
Multi-Object Spectrograph (WFMOS), proposed for the Subaru telescope, would
target galaxies at z 1.3 and
Lyman-break galaxies at 2.5
z 3.5. The
Physics of the Accelerating Universe (PAU) is a Spanish project to
deploy a wide-field camera with a large number of narrow filters to
measure coarse-grained galaxy spectra out to z = 0.9.

Finally, the proposed Square Kilometer Array (SKA), an array of radio
antennas with unprecedented collecting area, would probe dark energy
using baryon acoustic oscillations and weak lensing of galaxies via
measurements of the 21-cm line signature of neutral hydrogen (HI). The
Hubble Sphere Hydrogen Survey (HSHS) aims to carry out a 21-cm BAO
survey on a shorter timescale.

Three of the proposed space projects are candidates for the Joint Dark
Energy Mission (JDEM), a joint mission of the U.S. Department of Energy
(DOE) and the
NASA Beyond Einstein program, targeted at dark energy science.
SuperNova/Acceleration Probe (SNAP) proposes to study dark energy using a
dedicated 2-m class telescope. With imaging in 9 optical and
near-infrared passbands and follow-up spectroscopy of supernovae, it is
principally designed to probe
SNe Ia and weak lensing, taking advantage of the excellent optical image
quality and near-infrared transparency of a space-based platform.
Fig. 17 gives an illustration of the statistical
constraints that the proposed SNAP mission could achieve, by combining
SN and weak lensing observations with results from the Planck CMB
mission. This forecast makes use of the Fisher information matrix
described in the Appendix
(Section 11.2). The Dark Energy Space
Telescope (DESTINY) would use a
similar-size telescope with a near-infrared grism spectrograph to study
supernovae. The Advanced Dark Energy Physics Telescope (ADEPT) is a
spectroscopic mission with the primary goal of constraining dark energy via
baryon acoustic oscillations at z ~ 2 as well as supernovae. Another
proposed mission within the NASA Beyond Einstein program is Constellation-X,
which could observe X-ray clusters with unprecedented sensitivity.

Figure 17. Illustration of forecast
constraints on
dark energy parameters. Shown are 68% C.L. uncertainties for one
version of the proposed SNAP experiment, which combines a narrow-area
survey of 2000 SNe to z = 1.7 and a weak lensing survey of 1000
sq. deg. Left panel: Constraints in the
M -
w plane, assuming constant w;
the vertical axis can also be interpreted as the pivot value
wp for a time-varying equation of
state. Right panel: Constraints in the
w0 - wa plane for time-varying
dark energy equation of state, marginalized over
M for a
flat Universe.

There is one European Space Agency (ESA) mission nearing launch and two
concepts under study. The Planck mission, planned for launch in late
2008, in addition to pinning down other cosmological parameters
important for dark energy, will detect thousands of galaxy clusters
using the SZE. Dark Universe Explorer (DUNE) and SPACE are optical
missions to study dark energy using weak lensing and baryon acoustic
oscillations, respectively. Finally, the extended ROentgen Survey with
an Imaging Telescope Array (eROSITA), a German-Russian collaboration, is
a planned X-ray telescope that will study dark energy using the
abundance of X-ray clusters.